SQUID gets to core of corrosion

Physicists at Vanderbilt University have developed a new remote sensing technique said to detect corrosion hidden deep within metal joints.

Physicists at Vanderbilt University have developed a new remote sensing technique said to detect corrosion hidden deep within metal joints where conventional electrochemical-detection methods fail.

According to a 1996 study by the Battelle Institute, corrosion in roads, bridges, passenger and freight railway systems, pipelines, harbours, airports, water treatment plants, solid waste disposal facilities and chemical processing plants may cost as much as $300 billion per year in the US alone.

About a third of this deterioration can be prevented using available means, the study estimates. But corrosion is not limited to places where it can be detected with conventional methods as it also occurs on hidden surfaces where it is difficult to detect and study.

John W Wikswo, A. B. Learned Professor of Living State Physics, and research associate Grant Skennerton claim they have successfully used a super-sensitive piece of microelectronics, called a Superconducting QUantum Interference Device (SQUID). The device is said to be able to detect subtle changes in magnetic field strength that are generated when small amounts of metal corrode.

‘Relative to their sensitivity, a tremendous amount of magnetic flux is generated when a small amount of metal corrodes,’ said Wikswo. Moreover, the SQUID does not need to make physical contact with a metal specimen to detect the presence of corrosion.

The study specifically tested the SQUID’s ability to image the magnetic fields associated with ongoing, hidden corrosion in metal samples removed from ageing military aeroplanes.

The technique, however, does have an important limitation. To date, all of the studies have been conducted in a laboratory on small aluminium samples inside a magnetic shield. The shield keeps out the ambient magnetic field that would interfere with the delicate measurements.

‘Unless we can find a way to compensate for the complex and time-varying contributions of ferromagnetic contamination, ferromagnetic fasteners, and the Earth’s magnetic field, we will not be able to use SQUIDs to detect corrosion in the field,’ said Wikswo.

An example of the role that SQUIDs can play in the fight against corrosion is an experiment the physicists did to test the relationship of the salt content of water to the corrosion of critical lap joints used in aircraft fuselages.

They exposed a set of lap joints to increasingly corrosive environments—humid air, distilled water, and then three increasingly concentrated salt solutions—and measured the magnetic fields associated with the corrosion caused by each environment.

‘We did not expect that humid air would generate much corrosion, and this was borne out in our experiments,’ said Wikswo. ‘There was a significant increase in corrosion going from humid air to distilled water that we also expected.’

When they switched to the salt solutions they found that there was no significant increase in corrosion following the increases in the concentration of salt solutions, said Wikswo.

Since then other investigators working on the project have confirmed this result using more conventional corrosion measurements. ‘This indicates that the chemistry within a lap joint might be different from that for exposed aluminium surfaces, where corrosion has a documented dependence on salt concentration,’ concluded Wikswo.